Code division multiple access (CDMA) is a channel access method used by various radio communication technologies. CDMA uses a transmission bandwidth that is usually several orders of magnitude larger than the minimum required signal or information bandwidth. The power of each user's signal is spread over a wide bandwidth. This results in low power spectral density, and thereby, the interference to another narrow band signal occupying the same frequency range is reduced. It also makes the presence of the signal less detectable.
A digital signal from each user in a CDMA system is modulated with a pseudo-noise (PN) binary sequence that is unique to that particular user. This modulation causes the spreading over the wide bandwidth. Each PN sequence appears random to an observer but can be reproduced in a deterministic manner by an intended receiver. Any two PN sequences are made substantially orthogonal to each other but some degree of cross correlation still exists. The mutual interference in the same frequency range of multiple users is greatly reduced in a CDMA system. This orthogonality allows multiple access within the same frequency spectrum, and makes CDMA systems less vulnerable to intentional or unintentional interference.
Detection of a CDMA system usually involves cross-correlation with a locally generated version of the PN sequence. A desired user signal is generally detected by cross correlating with the exact same PN sequence that is uniquely assigned to that particular user by the system. Typical CDMA receivers continually demodulate and correlate a received signal to a reference signal. However, this consumes time and processor resources. Moreover, in a blind frequency scanning application, the possible frequency centers for CDMA signals may not be known. Performing correlation filtering at all possible locations is time consuming and processor resource intensive.
A conventional reception timing detection method (chip synchronization) is described in the document titled “Principle of Spread Spectrum Communication” by Andrew J. Viterbi, published on April, 1995, Chapter 3, pp. 39-66. The signal that has been spread with a spreading sequence as a pseudo random code is captured at two phases, initial searching synchronous and tracking synchronous. In the initial searching synchronous method, serial search is executed by sliding the reception timing by ½ chip interval until the correlation power exceeds a given threshold value. In the tracking synchronous method, called as early-late gate or delay lock loop (DLL), the timing is finely adjusted by obtaining the correlation power at the timing ahead the reception delay time by Δt and the correlation power at the timing behind the reception delay time by Δt so as to make the difference zero.
Another approach is directed to correlation using only selected chip position samples, as disclosed in U.S. Pat. No. 6,996,162. A wireless communications signal is received, wherein the signal includes a first synchronization channel component. A synchronization channel value is correlated to the signal to produce a plurality of correlation samples in response to a correlation between the synchronization channel value and the signal. The plurality of correlation samples is compared to a threshold, and stores selected ones of the plurality of correlation samples a first set of correlation samples that exceed the threshold and are within a first time sample period. Each of the correlation samples in the first set has a corresponding sample time relative to the first time sample period. A second set of correlation samples is combined with the first set of correlation samples.
Yet another approach for detecting CDMA signals is disclosed in U.S. Pat. No. 5,872,776. A multiuser detector for a CDMA system for real-time processing uses interference information of cross correlation from the signals of different users is processed to enhance the signal-to-noise ratio by suppressing the interference noise in a multiple-stage processing. A sampling processor is implemented to obtain at least two sets of data for each user with different integration times. The operation of the detector is dynamically adjusted to the operating condition of the CDMA system in a near real-time manner.
Even in view of the above approaches for processing CDMA signals, there is still a need to improve on such processing so that the less processor resources are needed.